of Europe, while those near the Atlantic coast are more like those of Asia. Less than a score of insects are known to live in the sea.

The distribution of fishes is bounded by narrower limits than that of other animals. A few tribes may be called cosmopolitan, as the sharks and herrings; but the species are local. Size does not appear to bear any relation to latitude. The marine forms are three times

as numerous as the fresh-water. The migratory fishes of the northern hemisphere pass to a more southern region in the spring, while birds migrate in the autumn.

Living reptiles form but a fragment of the immense number which prevailed in the Middle Ages of geology. Being less under the influence of man, they have not been forced from their original habitats. None are arctic. America is the most favored spot for frogs and salamanders, and India for snakes. Australia has few batrachians, and two thirds of its snakes are venomous. In the United States, only about one eighth of the species are venomous. Frogs, snakes, and lizards occur at elevations of over fifteen thousand feet. Crocodiles, and

most lizards and turtles, are tropical.

Swimming birds, which constitute about one fourteenth of the entire class, form one half of the whole number in Greenland. As we approach the tropics, the variety and number of land birds increase. Those of the torrid zone are noted for their brilliant plumage, and the temperate forms for their more sober hues, but sweeter voices. India and South America are the richest regions. Hummers, tanagers, orioles, and toucans are restricted to the New World. Parrots are found in every continent except Europe; and woodpeckers occur in every region, save in Australia.

The vast majority of mammals are terrestrial; but cetaceans and seals belong to the sea, otters and beavers

delight in lakes and rivers, and moles are subterranean. As of birds, the aquatic species abound in the polar regions. Marsupials inhabit two widely separated areas - America and Australia. In the latter continent they constitute two thirds of the fauna, while nearly all placental mammals, except bats and a few rats and squirrels, are wanting. Excepting a few species in South Africa and South Asia, edentates are confined to tropical South America. The equine family is indigenous to South and East Africa and Southern Asia, while their fossil remains are abundant in both North and South America. In North America, rodents form about one half the number of mammals; there are very few species in Madagascar. Ruminants are sparingly represented in America. Carnivores flourish in every zone and continent. The prehensile-tailed monkeys are strictly South American; while the anthropoid apes belong to the west coast of Africa, and to Borneo and Sumatra. Both monkeys and apes are most abundant near the equator; in fact, their range is limited by the distribution of palms.

CHAPTER XXV

THE ORIGIN OF ANIMAL SPECIES

THE origin of the immense number of species of plants and animals inhabiting the earth has been a matter of speculation among naturalists and philosophers for many centuries. One theory has held that each species was created separately, while the other, known as the Theory of Evolution, maintains that living forms are derived by natural processes of descent from species that inhabited the earth in earlier times; that is, the ancestral forms became extinct owing to changing conditions of climate, food supply, enemies, and other factors, and their descendants in the course of many generations have become modified in bodily structure and function, these changes leading to the development, or evolution, of the numerous species now living. The evidence in favor of the latter theory is so strong that it is now accepted by scientific men as the true explanation of the mode of origin of all known organisms.

Although the idea of evolution has been more or less definitely held by various naturalists since the time of Aristotle (384-322 B.C.), others, even as recently as Linnæus (1707-1778) and Cuvier (1768–1832), have insisted that species are immutable, or unchanging in characteristics. Bonnet (1720-1793) was the first among later zoölogists to suggest that variations of climate, nourishment, and other features of the environment might produce new species, and to use the term evolution in its modern sense; but he adduced no important facts to

support his theory, and it failed to meet with the approval of his contemporaries. Lamarck (1744–1829) afterward adopted this view, collected many facts in its favor, and also advanced the hypothesis, in 1801, that the use and disuse of organs would cause structural modifications in them, producing either increased development or atrophy of parts. These modifications, being inherited by successive generations, would eventually become characteristic of new species thus evolved from the older ones. Lamarck's theory was opposed by Cuvier, the greatest comparative anatomist and paleontologist of his time, who insisted that, if the theory were true, there ought to be among fossils transition forms connecting the extinct with the living species, but that no such forms were known, nor could a process be suggested by which transition could take place. Under Cuvier's leadership the belief became current among geologists that the earth has passed through a series of catastrophes or cataclysms which destroyed all living things, and that it has successively been repeopled with new forms quite unlike those which had perished. The Lamarckian theory passed into obscurity, and was not seriously considered again until it was brought forth for comparison with Darwin's theory of natural selection. The opinions of geologists regarding cataclysms underwent a change after Hutton (1726-1797) urged that in order to understand how the present condition of the earth came about, the changes now taking place must be studied. This view was later vigorously upheld and extended by Lyell (1797-1875), who contended that cataclysms have never occurred, but that the earth has gradually reached its present state through the action of natural forces which are still in operation. Thus the way was prepared for the appearance of the theory which, elaborated and maintained by numerous observa

tions, was propounded by Charles Darwin (1809-1882) in his "Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life," published in 1859. Darwin had served as naturalist on the British exploring ship Beagle on a five years' cruise (1832-1837) around the world, and "was much struck with certain facts in the distribution of the organic beings inhabiting South America, and in the geological relations of the present to the past inhabitants of that continent." After his return home, twenty additional years were spent in collecting facts, making further observations and experiments, and in pondering the theory before he ventured to publish his results and to state what he regarded as the factors concerned in the process of evolution. A similar conclusion had been reached, simultaneously and independently, by Alfred Russell Wallace (1823- ), who had travelled extensively in South America and the Malay Archipelago, and, like Darwin, had become convinced of the certainty of evolution, and sought for its explanation.

As held by Darwin, the theory of evolution, together with the causes of the process, may be briefly stated as follows:

(1) Organisms tend to produce a great many more offspring than can survive. Linnæus showed that the number of living descendants of an annual plant which produced only two seeds each year would, at the end of twenty years, be over a million. There is, however, no plant known to be so unproductive. With reference to the elephant, regarded as the slowest of breeders, producing at the age of thirty a pair of young, and a pair every thirty years thereafter, and living to be one hundred years old, Darwin computed that at the end of 750 years there would be about nineteen million living elephants all descended from the first pair. Individual insects lay